Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Most types of internal combustion engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
Many transmissions utilize friction clutches and brakes which are engaged in different combinations to establish different power flow paths having different speed ratios. One type of commonly used friction clutch is a wet multi-plate clutch. A clutch pack includes a set of friction plates splined to one component and interleaved with a set of separator plates splined to a different component. To engage the clutch, pressurized fluid is supplied to an apply chamber forcing a piston to squeeze the friction plates between the separator plates. Friction between the friction plates and separator plates prevents relative rotation coupling the two components to each other. When the fluid pressure is reduced, a return spring forces the piston away from the clutch pack removing the normal force such that relative rotation is possible with minimal drag. Wet multi-plate clutches rely on a supply of transmission fluid to the friction material on the friction plates. This fluid serves several purposes, including modifying the friction characteristics of the material and removing excess heat.